Such a method of processing data protected by a coding is described in the publication "Federal Information Processing Standards Publication" (FIPS PUB 46) of Jan. 15th 1977, of the National Office of Standards of the U.S. Ministry of Commerce. When such a processing is applied to the data utilized in a microcircuit card, the processing must be as rapid and as compact as possible, the code length admissible in a microcircuit card being strongly limited. One of the steps of the coding, which step will be designated from now on as "Step SP", consists in a compression transformation S by look-up tables designated by Ti (corresponding to respective selection functions Si) followed by an irregular permutation P.
Conventionally, the compression S and the permutation P are effected in sequence (the permutation P being applied to the result of the compression S), which on the one hand necessitates a prohibitive processing time and, on the other hand requires a great code length and hence, an exaggerated storage occupation. In fact, the conventional method of carrying out a permutation of words, irrespective of their type, consists in taking each bit of the input word, calculating its position in the output word (generally by means of a table when the permutation is entirely irregular, as in the present case) and positioning therein the value of the bit considered. Since this processing is effected bitwise, it is clear that the cost of this method is proportional to the number of bits to be permuted. As to the execution of the transformation of compression S, the input word is decomposed into elementary words Wi, used as an index in the tables Ti, so as to obtain) the output word the tables Ti supplying (after concatenation of the different values stored in the tables Ti) the output word based on the elementary words Wi.